NEW YORK — It’s a vexing puzzle about the Boston Marathon bombings: The younger of the two accused brothers hardly seemed headed for a monumental act of violence. How could he team up with his older brother to do this?

Nobody knows for sure, but some experts in sibling research say the powerful bonds that can develop between brothers may have played a role.

Additional Photos

This April 15, 2013, photo provided by Bob Leonard shows bombing suspects Tamerlan Tsarnaev, 26, in black hat, and his brother, Dzhokhar A. Tsarnaev, 19, in white hat, approximately 10-20 minutes before the blasts that struck the Boston Marathon. Some experts in sibling research say the powerful bonds that can develop between brothers may have played a role in the bombings. AP

Tamerlan Tsarnaev, who died last week at age 26 in a shootout with police, and his 19-year-old sibling Dzhokhar are hardly the first brothers involved in criminal acts. Three pairs of brothers were among the 9/11 terrorists, for example, and three brothers were convicted in 2008 for planning to attack soldiers at Fort Dix in New Jersey.

“There are a lot of criminal enterprises where you have brothers involved,” said James Alan Fox, a criminologist at Northeastern University in Boston. “It is almost always the older brother who is the leader. … Typically the younger brother looks up to the older brother in many ways.”

Friends and relatives paint markedly different pictures of the Tsarnaev pair. Tamerlan could be argumentative and sullen, saying at one point he hadn’t made a single American friend since immigrating years earlier, and he was arrested in 2009 for assault and battery on a girlfriend before those charges were dismissed. Dzhokhar appears to have been well-adjusted and well-liked in both high school and college.

Tamerlan seemed to be the dominant sibling in the family.

“He was the eldest one and he, in many ways, was the role model for his sisters and his brother,” said Elmirza Khozhugov, 26, the ex-husband of Tamerlan’s sister, Ailina. “You could always hear his younger brother and sisters say, ‘Tamerlan said this,’ and ‘Tamerlan said that.’ Dzhokhar loved him. He would do whatever Tamerlan would say.”

Federal officials say their initial questioning of Dzhokhar suggests both brothers were motivated by a radical brand of Islam without apparent connections to terrorist groups. Their uncle, Ruslan Tsarni, has blamed their alleged bombing partnership on Tamerlan, saying Dzhokhar has been “absolutely wasted by his older brother … He used him … for what we see they’ve done.”

Research shows that older brothers can have a direct influence on younger ones, says Katherine Conger, an associate professor of human development and family studies at the University of California-Davis.

“Sometimes it’s through having a high quality relationship. So they spend time together, they enjoy doing things together and kind of hang out,” she said. But other times, she said, it’s through coercion and threats.

Studies show that children and adolescents can be influenced toward theft, vandalism and alcohol use by their older siblings. The influence is even more pronounced when parenting is harsh, inconsistent or absent, and when the two siblings share the same friends, experts said.

So how might that apply to the Tsarnaev brothers? There are several reasons to be wary about extrapolating the research to this case: So little is known about the brothers’ family lives and other details. And most sibling research examines more ordinary infractions occurring in Western cultures – not the extreme behavior believed carried out by the Tsarnaevs, who shared both an American culture and an ethnic Chechen background.

Still, from the sketchy details in press reports, some experts said it makes sense that Tamerlan could have had a major influence on his younger brother. That may have been through a close relationship or coercion, Conger said. “It’s really hard to know.”

Lew Bank of Portland State University in Oregon said Dzhokhar may have looked up to his older brother and wanted to please him. “It was very likely exciting for the younger brother to be so intensively at work with his big brother at something that seemed so important to them both,” Bank said.

The relationship may have intensified when both parents left the country within the past year or so, leaving Tamerlan as Dzhokhar’s dominant family member, he said. Tamerlan may have taken on a father-like role, said Avidan Milevsky of Kutztown University of Pennsylvania.

But for Laurie Kramer of the University of Illinois, a key question remains. Why couldn’t Dzhokhar tell his older brother, “This isn’t right, this isn’t acceptable?” she asked. “This seems to be a case where a little bit more sibling conflict might have been useful.”

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